PAPI is a highly reliable and accurate aid below 200 ft. At lower altitudes you may not wish to use PAPI to establish the visual glidepath (long body aircraft, GS Ax position, or PAPI location), but it remains a useful aid for assessing the trend of the approach and detecting sudden changes in vertical position, i.e. windshear.
For general info and description, see pages 194/5, and 253-> in
CAP 168.
Also pages 9 – 12 etc in
CAP 637 Visual Aids Handbook.